Spring Data JPA + Hibernate：如何使用抽象超类上的存储库进行对象检索？

Question

I am asking as a follow up to this question . I am doing the exact same thing and it does not work, meaning it still gives an Exception indicating attempting to instantiate an abstract class:

org.springframework.orm.jpa.JpaSystemException: org.hibernate.InstantiationException: Cannot instantiate abstract class or interface

The only difference is that I do not set my classes as implementing the Serializable interface, but I figured that is probably not the reason, so I suspect the Spring and Hibernate version.

so basically the abstract class looks something like this:

@Entity
@Inheritance(strategy = InheritanceType.JOINED)
@Table(name="activities")
@DiscriminatorColumn(name="type", discriminatorType=DiscriminatorType.STRING)
public abstract class Activity
{
    @Id
    @Column(length=Fields.ID_FIELD_SIZE)
    protected String id;
    ....

and two sub-classes

@Entity
@Table(name="a_activities")
@DiscriminatorValue("A")
public class AActivity extends Activity
{
...

and

@Entity
@Table(name="b_activities")
@DiscriminatorValue("B")
public class BActivity extends Activity
{
...

When using the ActivityRepository to fetch an Activity by Id it gives the mentioned exception.

@Qualifier("activity-repository")
public interface ActivityRepository extends JpaRepository<Activity, String>
{
}

I am using Spring Data 1.2.0 and Hibernate 4.1.7.

Sorry for asking as a separate question but I could not comment on the original one

Answer 1

I managed to resolve this issue. The main reason this was happening is that this hierarchy was introduced into our existing system which means we had data in the database before this change. The Activity class was a concrete class before and the tables for the sub-classes did not exist. When adding this hierarchy we created the tables for the sub-classes; however, what I missed was the need to insert an entry for each Activity object in the proper table for the sub-class which it instantiates.

I tried this on one entry and I managed to successfully fetch that Activity by Id. So we should run scripts to properly insert sub-class entries for all existing activity entries in the proper tables.

It turns out this answer already notes this point but somehow I noted the check on the Discriminator values but missed the need to check that each entry has a row in the corresponding sub-class table.